Re: [PATCH v2 4/5] mm: make vma cache SLAB_TYPESAFE_BY_RCU

[Vlastimil Babka <vbabka@suse.cz>]

On 11/12/24 20:46, Suren Baghdasaryan wrote:
> To enable SLAB_TYPESAFE_BY_RCU for vma cache we need to ensure that
> object reuse before RCU grace period is over will be detected inside
> lock_vma_under_rcu().
> lock_vma_under_rcu() enters RCU read section, finds the vma at the
> given address, locks the vma and checks if it got detached or remapped
> to cover a different address range. These last checks are there
> to ensure that the vma was not modified after we found it but before
> locking it. Vma reuse introduces a possibility that in between those
> events of finding and locking the vma, it can get detached, reused,
> added into a tree and be marked as attached. Current checks will help
> detecting cases when:
> - vma was reused but not yet added into the tree (detached check)
> - vma was reused at a different address range (address check)
> If vma is covering a new address range which still includes the address
> we were looking for, it's not a problem unless the reused vma was added
> into a different address space. Therefore checking that vma->vm_mm is
> still the same is the the only missing check to detect vma reuse.

Hi, I was wondering if we actually need the detached flag. Couldn't
"detached" simply mean vma->vm_mm == NULL and we save 4 bytes? Do we ever
need a vma that's detached but still has a mm pointer? I'd hope the places
that set detached to false have the mm pointer around so it's not inconvenient.